The present invention relates to lasers emitting single longitudinal and traverse mode radiation at selected wavelengths defined by a frequency comb, in particular stepwise tunable external cavity surface emitting semiconductor lasers pumped optically or electronically for use in spectroscopy, process control and optical communications. Particularly disclosed are laser designs, and manufacturing and assembly processes for frequency stable and rapidly tunable lasers for optical communications.
The use of lasers as part of a system for optical channel switching in a fiber optic transport network is known. However, existing systems utilize a multiplicity of individual lasers, each of which emits at a single frequency. A further problem is that current switching systems using single frequency lasers require extremely complex circuitry to transform a set of input signals to a set of output signals of different frequencies.
A significant enhancement of such laser based optical channel switching systems would be achieved by use of a laser having the following operating characteristics:
1) the laser is rapidly tunable to specific desired output frequencies, e.g. the frequencies of the ITU grid.
2) The laser provides random access to any particular output frequency (i.e. transmission channel).
3) The laser is reliable and consistent in the output frequency to which it is tunable over a long service life without requiring extensive servicing or a carefully controlled operating environment.
4) The laser is consistently receptive to an input signal since it must always tune to the correct output frequency (i.e. channel number).
5) The laser provides substantially uniform output power independent of the particular output frequency selected.
We have discovered a laser system which fulfills the above indicated performance requirements.
Typical lasers oscillate at a number of frequencies (i.e. wavelengths) that correspond to modes of their optical cavities. In certain applications, it is known to select a single such mode and to adjust its frequency by varying the cavity length or some other laser parameter. Methods of doing this are described in text books such as A. Yariv, Quantum Electronics, John Wiley and Sons, New York, 2nd edition, 1975 and M. D. Levenson and S. S. Kano, Introduction to Nonlinear Laser Spectroscopy Revised Edition, Academic Press, San Diego, 1988 and Demtroder, Laser Spectroscopy, Springer, Berlin, 1996.
However, the task of quickly switching the laser output frequency from one externally selected value to another externally selected frequencyxe2x80x94without undergoing laser action at intermediate frequenciesxe2x80x94poses significant technical difficulties. Known means of doing this alter some other significant laser parameter, such as output power, create an instability in the output frequency either before or after the frequency switch, and/or switch to an unwanted value which must then be homed-in on (actually adjusted to) the desired frequency, which requires time. In many optical communications and spectroscopy applications, delay in obtaining stable operation after a frequency switch is certainly undesirable and is frequently unacceptable.
Most semiconductor lasers generally follow two basic architectures. The first laser type has an in-plane cavity, and the second laser type has a vertical cavity, a so-called vertical-cavity surface-emitting laser or xe2x80x9cVCSELxe2x80x9d. If the optical resonance cavity is formed externally of the semiconductor structure (active region) i.e. one of the reflecting surfaces is physically separated from the active region, the laser is known as a vertical external cavity surface-emitting laser or xe2x80x9cVECSELxe2x80x9d.
Electrically pumped diode lasers are most frequently of the in-plane cavity type. Necessary optical feedback with the in-plane type is most frequently provided by simple cleaved-facet mirrors at each end of the optical cavity. The reflectance of such cleaved mirrors, while generally sufficient is not very high, and laser energy is thus emitted through the cleaved mirrors to the external ambient at opposed edges of the structure, giving rise to xe2x80x9cedge-emittingxe2x80x9d diode lasers. Such relatively simple structures are sometimes referred to as Fabry-Perot diode lasers. Epitaxial patterning of a grating pattern along a top surface of an edge-emitting diode laser can be provided to set a design wavelength, resulting in a so-called distributed feedback diode (xe2x80x9cDFBxe2x80x9d) laser.
In-plane electrically pumped lasers, such as DFB lasers, are typically single mode, and are also typically tunable continuously across some wavelength band from near-infrared into the visible light spectrum. Rapid tuning may be carried out by controlling the electrical pumping current, while slower tuning may be carried out by controlling the temperature of the laser via a heat sink and a thermal cooler/heater arrangement. Such in-plane lasers have known uses including optical wavelength absorption spectroscopy, storage, printing and telecommunications. In-plane lasers are frequently employed within telecommunications systems using optical fiber as the information transfer medium. Conventionally, multiple channels are carried through a single optical fiber, and it is therefore necessary when using a Fabry-Perot laser or DFB laser as the illuminating source to regulate the wavelength of the transmitting laser in order to stay on a selected channel.
In order to keep an in-plane diode laser tuned to a desired wavelength, current and thermal control loops must be provided to stabilize the laser at a desired wavelength, particularly as the laser ages during usage. Also, since there is no absolute wavelength stabilization within these in-plane lasers, the emission wavelength may drift, absent careful feedback control, during usage and over the lifetime of the laser. This tendency to drift or change emission characteristics with temperature and over time puts stringent conditions on the materials and control systems used to make the laser.
One known drawback of in-plane diode lasers, and most particularly the Fabry-Perot type, is that they manifest a tendency to mode-hop. Mode-hopping basically means that for a given pumping current, the laser can unexpectedly hop to a completely different mode (wavelength). As the current is increased, there are wavelengths at which the mode hopping (wavelength jumping) becomes uncontrollable. Moreover, diode lasers may manifest a hysteresis, in that mode hopping may occur at different wavelengths as the control current is increased as compared to when the control current is decreased. Another drawback of in-plane diode lasers is that output power is inextricably intertwined with active region temperature and pumping current. Another issue with in-plane diode lasers is that the transverse optical beam profile is typically elliptical rather than circular and has high divergence, increasing the complexity of coupling the laser energy into an optical fiber, or coupling the laser waveguide mode to an external cavity mode.
Dense wavelength division multiplexing (DWDM) for optical fiber telecommunications applications requires optical transmitters that can be tuned to any frequency in the standard ITU grid (wavelength comb) with a relative frequency error not greater than ten percent of the ITU channel spacing. This requirement implies that an optical transmitter laser has excellent frequency stability as well as broad tunability. For a 12.5 GHz channel spacing, the transmitter must have 1.25 GHz of absolute accuracy and frequency (wavelength) stability. Such control of the laser frequency cannot be achieved with existing DFB lasers without complex electronic control and frequently carried out diagnostics. Furthermore, compensation algorithms must be developed in the laser control system to handle the DFB""s known aging processes, which is often unpredictable.
A desirable characteristic of an ideal DWDM optical transmitter is that a single laser can cover all of the DWDM channels, and that it can be reliably and reproducibly set to any desired one of the standard channel frequencies. Current laser sources have only a limited tuning range, which covers only a fraction of the full ITU grid. Known DFB lasers have limited tunability; and the temperature tuning coefficient of telecom DFB lasers is typically 0.09 nm/xc2x0 C. For a DFB laser thermal operating range of xc2x120xc2x0 C., or 40xc2x0 C. total temperature differential, one DFB laser could only be expected to cover a wavelength range of 3.6 nm (or about 460 GHz, representing only four channel coverage with 100 GHz channel spacing or 36 channel coverage with 25 GHz spacing) even if the necessary accuracy in wavelength could be achieved.
In addition, DFB lasers only have about 35 to 45 dB of side mode suppression. If the side modes are not sufficiently controlled, the laser may excite two or three adjacent communications channels, resulting in unwanted interference. Because of these drawbacks, the telecommunications industry has recently turned to VCSELs.
Micro-cavity VCSELs include semiconductor structures which have multiple layers epitaxially grown upon a semiconductor wafer/substrate, typically Gallium Arsenide or Indium Phosphide. The layers comprise semiconductor or dielectric Bragg mirrors which sandwich layers comprising quantum well active regions. Within the VCSEL, photons emitted by the quantum wells bounce between the mirrors and are then emitted vertically from the wafer surface. The VCSEL type laser naturally has a circular dot geometry with lateral dimensions of a few microns. The emitting aperture of a few microns facilitates direct-coupling to optical fibers or other simple optics, since the narrow aperture typically supports only a single lateral mode (TEM00) of the resulting optical waveguide, but is sufficiently wide to provide an emerging optical beam with a relatively small diffraction angle. A 1.3 micron VCSEL is said to have been developed by Sandia National Laboratories in conjunction with Cielo Communications, Inc. According to a news report, xe2x80x9cThis new VCSEL is made mostly from stacks of layers of semiconductor materials common in shorter wavelength lasersxe2x80x94aluminum gallium arsenide and gallium arsenide. The Sandia team added to this structure a small amount of a new material, indium gallium arsenide nitride (InGaAsN), which was initially developed by Hitachi of Japan in the mid 1990s. The InGaAsN causes the VCSEL""s operating wavelength to fall into a range that makes it useable in high-speed Internet connections.xe2x80x9d (xe2x80x9cxe2x80x98First everxe2x80x99 1.3 micron VCSEL on GaAsxe2x80x9d, Optics.Org Industry News, posted Jun. 16, 2000). One of the characteristics of micro-cavity VCSELs is that the laser cavity is formed entirely within the semiconductor structure. A drawback of such VCSELs is that they do not generate much power, on the order of 3 mW for a small aperture of 5 xcexcm, for example. Also, there is transverse spatial hole burning between the transverse modes above about 3 mW.
As mentioned above, if a cavity is formed which is external to the VCSEL semiconductor structure having a quantum well active region, it is known as an external (extended) cavity laser (ECL). The semiconductor structures used in ECLs have been typically Fabry-Perot lasers having one facet antireflection coated so as not to interfere with external cavity operation. However, the gain medium can also be achieved using a SOA, (i.e. a semiconductor optical amplifier which is typically an edge-emitting semiconductor gain structure which however does not include the mirrors needed to convert it into a laser oscillator) or half of a VCSEL.
A semiconductor optical amplifier (SOA) is a device that amplifies an input signal of optical origin. The amplification factor is typically high ( greater than 20 dB). A SOA amplifies light as it propagates through a waveguide made of semiconductor material. A SOA is typically less than 1 mm in length. It amplifies light through stimulated emission Oust as a laser produces radiation). In essence, a SOA is a Fabry-Perot laser without feedback, having optical gain when the amplifier is pumped (optically or electrically) to create a population inversion leading to stimulated emission. The optical gain of wavelength and signal intensity depends on the SOA design and medium. However, a SOA can form the gain medium of a laser.
Typically a SOA has two AR (anti-reflection) coated facets. In some cases, the waveguide is defined by a ridge that has non-normnal incidence to the facets to further reduce the effective facet reflectivity. The SOA design can lead to facet reflectivities that are 10 to 100 times smaller than in a Fabry-Perot laser. Were a SOA designed to have only one AR coated facet with non-normal incidence and normal incidence of the waveguide at the other facet, then it would form an active mirror. The non-normal incidence of the facet could either be cleaved (30% reflectivity) or HR (high reflectivity) coated. SOAs can also be designed to have a square waveguide structure which lends itslf to a circular Gausian beam.
In both VCSEL and SOA cases, one creates a so-called xe2x80x9cactive mirrorxe2x80x9d (also referred to as an xe2x80x9camplifying mirrorxe2x80x9d) that defines one of the laser cavity reflectors but also provides the gain medium for the laser. The gain in this active mirror results from either electrical or optical excitation (pumping) of carriers that recombine in the quantum wells to create photons. The external cavity defines the coherence properties (wavelength) of these photons. If the gain medium is half a VCSEL, the external cavity version is referred to as a VECSEL. If the gain medium is a SOA the laser is called ECSAL. In general, we will refer to both such types of laser as a STECAM, a stepwise tunable, external cavity, active mirror laser.
A VECSEL based active mirror is an epitaxially grown semiconductor body, typically a few microns thick, which comprises a multiple quantum well active gain region sandwiched between a Bragg mirror grown on a semiconductor substrate and a capping layer. The active mirror may also have an antireflection coating that is either epitaxially grown or dielectrically deposited. An external cavity is then formed by a second, passive mirror that forms a stable resonator with the active mirror. Such an external cavity can either be a high reflectivity dielectric concave mirror or a plano/plano mirror with an intracavity refocusing element such as a lens. One example of an optically-pumped VECSEL is described in Published International Patent Application WO 00/10234, entitled xe2x80x9cOptically-Pumped External-Mirror Vertical-Cavity Semiconductor-Laserxe2x80x9d, the disclosure thereof being incorporated herein by reference. The disclosed VECSEL includes an epitaxially-grown semiconductor structure having a multiple-layer mirror structure integrated with a multiple-layer quantum-well structure which provides a gain medium, and an external mirror forming a resonant cavity with the integrated semiconductor multilayer mirror. Optical pumping radiation is directed at the quantum-well structure via an outermost (top layer) and is absorbed by the quantum-well and pump-absorbing layers. The quantum-well layers release photons in response to the pumping energy, and the external cavity is dimensioned to result in laser energy output at an approximate 976 nm wavelength in response to pumping energy at a wavelength of approximately 808 nm. Because this VECSEL operates at wavelengths below 1.1xcexc in the near infra-red spectrum, the active gain medium is made to be aluminum-free, since aluminum ions tend to diffuse in GaAs/AlGaAs lasers. Accordingly, the quantum-well and pump-radiation absorbing layers are aluminum-free layers of alloys of gallium arsenide and indium gallium arsenide phosphide (GaAs/InGaAsP). However, a drawback of the VECSEL described in this published International Patent Application is the absence of any wavelength tuning mechanism enabling adjustment of the laser emission wavelength.
Other VECSELs are described, inter alia, in a paper by Sandusky and Brueck, entitled: xe2x80x9cA CW External-Cavity Surface-Emitting Laserxe2x80x9d, IEEE Photonics Tech. Ltrs., Vol. 8, No. 3, March 1996, pp. 313-315; and, in a paper by Kuznetsov, Hamimi, Sprague, and Mooradian, entitled: xe2x80x9cHigh Power ( greater than 0.5-W CW) Diode-Pumped Vertical-External-Cavity Surface-Emitting Semiconductor Lasers with Circular TEM00 Beamsxe2x80x9d, IEEE Photonics Tech. Ltrs., Vol. 9, No. 8, August 1997, pp. 1063-1065.
Co-inventors Gamache and Kachanov of the present invention have previously reported that an optically pumped multiple-quantum-well (xe2x80x9cMQWxe2x80x9d) VECSEL is an excellent candidate for use in high sensitivity intracavity laser absorption spectroscopy (xe2x80x9cICLASxe2x80x9d) in xe2x80x9cHigh-sensitivity intracavity laser absorption spectroscopy with vertical-external-cavity surface-emitting semiconductor lasersxe2x80x9d, Optics Letters, Vol. 24, No. 12, Jun. 15, 1999, pp. 826-828. In the ICLAS method an absorbent analyte is placed inside an external cavity of a broadband laser with homogeneously broadened gain. An L-shaped cavity was formed by the integrated Bragg mirror, an external folding mirror having a 150 mm radius of curvature, and a flat output coupler having 0.5 percent transmission placed at the end of a one meter arm of the cavity. The angle between the two arms was reduced to approximately 7 degrees to reduce astigmatism. A 500 mm long intra-cavity absorption cell with Brewster-angle windows and containing an analyte material was placed in the long arm. Generation time was controlled by an optical chopper that interrupts or starts the pump radiation beam and by an acousto-optic modulator that is triggered after an adjustable generation delay time. Further work by these authors with VECSELs in the field of spectroscopy is reported in a paper by Garnache, Kachanov, Stoeckel and Houdre entitled: xe2x80x9cDiode-Pumped Broadband Vertical-External-Cavity Surface-Emitting Semiconductor Laser: Application to High Sensitivity Intracavity Laser Absorption Spectroscopyxe2x80x9d, JOSA-B-B, Vol. 17, No. 9, September 2000, pp. 1589-1598. The disclosures of these two articles are incorporated herein in their respective entireties by this reference thereto.
An intra-cavity etalon and a Lyot filter were said by Holm et al. to stabilize VECSEL radiation at a single wavelength in xe2x80x9cActively Stabilized Single-Frequency Vertical-External-Cavity AlGaAs Laserxe2x80x9d, IEEE Photonics Technologzy Letters, Vol. 11, No. 12, December 1999.
One approach for tuning a VECSEL is described in a note by D. Vakhshoori, P. Tayebati, Chih-Cheng Lu, M. Azimi, P. Wang, Jiang-Huai Zhou and E. Canoglu entitled, xe2x80x9c2 mW CW single mode operation of a tunable 1550 nm vertical cavity surface emitting laser with 50 nm tuning rangexe2x80x9d, published in Electronics Letters, Vol. 35, No. 11, May 27, 1999, pp. 900-901, the disclosure thereof being incorporated herein by reference. The described laser was grown epitaxially upon an indium phosphide substrate and has a cavity formed by a distributed Bragg reflector (DBR), a multiple quantum well (MQW) active gain region, and an external dielectric membrane mirror at a relatively short (xcx9c7xcexc) distance from the active gain region. Because the VECSEL laser cavity is so short, only one cavity mode can fit into the bandwidth of the MQW gain structure. Cavity length can be changed by applying a potential difference between the dielectric membrane and the ambient supporting structure, thereby applying an electrostatic force to the membrane mirror and causing its curvature (and hence the cavity length) to change. Changing the cavity length shifts the cavity resonance frequency which thereby results in laser frequency tuning. The VECSEL is optically pumped by a 980 nm diode laser which can be epitaxially grown below the DWDM laser. The authors and an associated company, Coretek, have reported continuous-tuning of this VECSEL over a range of about 50 nm, which is more than 10 times the tuning range of a typical DFB laser. This Coretek VECSEL is said to have a high quality TEM00 transverse mode and more than 50 dB of side mode suppression.
However, the Coretek VECSEL does not appear to meet the DWDM telecom requirements. The micro-machined membrane mirror must be flexible in order to move the required tuning distance, and is therefore necessarily sensitive to external perturbations or vibrations and also can become self-excited into undesirable vibrational modes by actuation. This system is also complex to produce, with the evident difficulties of a multilayer epitaxial structure being compounded by the need to form, align and attach a precision micro-machined membrane external mirror. Thus, Coretek type VECSELs would be challenging to manufacture at a reasonable cost and yield in mass production. Furthermore, a complex feedback control system would be required to maintain membrane mirror position, thereby limiting absolute frequency set point stability and reproducibility in laser tuning.
Caprara et. al. (e.g. U.S. Pat. Nos. 5,991,318; 6,097,742 and 6,167,068) have described a very large, high-power VECSEL with intra-cavity harmonic generation crystals producing output radiation at 488 nm, (well below current telecommunication wavelengths). Since such a harmonic generation crystal creates loss for the laser mode being converted to a shorter wavelength, an additional intra-cavity wavelength control element is described. The fixed element described is a Brewster-angle birefringent plate. Such a tuning element requires mechanical rotation for adjustment and thus cannot provide rapid tuning. Moreover, mechanical adjustment causes energy to build up in the successive modes traversed by the filter transmission maximum.
Telle and Tang (Applied Physics Letters 24, 85-87 (1974) have described an electro-optic frequency selective filter for dye lasers that might be capable of rapid tuning if sufficiently high voltage can be sufficiently rapidly applied. However, the multi-kilovolt potentials required by that filter are too high for practical telecommunication use, and the beam collimation required is not compatible with VECSEL type lasers. Other previously known tunable filter technologies have too much loss for use with surface emitting semiconductor gain media and/or transmit extra unacceptable frequencies. However, when used in conjunction with a higher gain medium such as a SOA, these filters (e.g. liquid crystals) can provide a suitable lower voltage tuning alternative.
From the foregoing description of the state of the art, it is apparent that a hitherto unsolved need has remained for a simplified, reproducible and widely tunable single mode MQW VECSEL or SOA based laser for optical fiber telecommunications which overcomes the limitations and drawbacks of the prior art approaches. Especially, there remains a need for a compact near infra-red laser system capable of switching quickly ( less than 10 xcexcs) among cavity modes spaced at xcx9c25-50 GHz from one another without producing unwanted frequencies.
One object of the present invention is to realize an optical fiber transmitter module including a single mode MQW VECSEL having a semiconductor structure with a homogeneously broadened active gain region and an external mirror spaced from the semiconductor structure by a spacer such that a cavity length is in a range of 0.5 mm and 50 mm and is chosen to create a laser frequency comb corresponding to a predetermined optical channel spacing arrangement.
Another object of the present invention is to realize a MQW VECSEL semiconductor structure formed by molecular beam epitaxy or metal organic chemical vapor deposition in a manner enabling removal of the semiconductor substrate, thereby overcoming limitations and drawbacks of prior approaches in which the substrate contributed to the presence of a Fabry-Perot etalon or other unwanted optical element.
Another object is to realize an optical fiber transmitter module including a SOA with homogeneously broadened and unpolarized active gain and an external mirror.
Yet another object of the present invention is to realize a laser with reproducible absolute emission wavelengths that correspond to standardized wavelength division multiplex (WDM) channel wavelengths, as used in optical fiber telecommunications networks, such that the laser steps from channel to channel and such that by design the emission wavelengths of this laser are ensured to hit any desired channel wavelength accurately and have channel separation with an accuracy better than ten percent of channel spacing.
One more object of the present invention is to realize a fiber optic transmitter having sidemode suppression in excess of 40 dB.
One further object of the present invention is to provide a VECSEL or ECSAL for use as a laser source within a wide variety of applications and environments including telecommunications test equipment.
Another object of the present invention is to provide a compact VECSEL or ECSAL device with axial cavity modes (i.e. axial mode frequencies) which correspond to pre-determined communications or spectroscopic channels, and which is capable of randomly switching among such channels in 1 millisecond or less.
Another object of the invention is to provide a single ECSAL able to access the entire C or L optical communications band.
Another object of invention is to provide a method to access the entire C or L optical communications band with a single laser device.
Another object of this invention is to provide a frequency agile laser module meeting all current requirements for DWDM optical fiber communications.
An object of the present invention is to provide an external cavity type laser having a fixed cavity length selected so that permitted laser modes match desired emission frequencies (e.g. the frequencies of the ITU grid).
One further object of the present invention is to realize a compact multi-quantum well (MQW) based optical transmitter with cavity modes which correspond to pre-determined communications or spectroscopic channels.
Another object of the invention is to realize an optical transmitter that emits a stable TEM00 beam at the frequency of a specific channel and which can be switched to another such channel by changing some convenient control parameter.
Another object of the invention is to provide an optical transmitter that is capable of randomly switching among such channels in 0.1 millisecond or less.
A preferred laser includes an intra-cavity, fast electro-optic tuning element providing minimum optical loss only at selected frequencies.
A preferred laser also includes a gain medium that is homogeneously broadened, coupled to a circular (Gaussian) external laser mode.
An optical fiber transmitter comprises an active mirror for emitting an information-carrying laser beam at a design wavelength and has an external cavity length defining a plurality of optical modes, each mode corresponding to a channel wavelength of an optical telecommunications system having plural optical channels. The active mirror (the mirror which is part of the gain medium) such as the semiconductor structure of an optical-pump-excited VECSEL or a SOA, has a homogeneously broadened multiple quantum well (MQW) active region wherein the gain curve exceeds cavity losses over a band which is less than mode-to-mode spacing, the gain region being tunable to step from a first mode to an adjacent second mode and to remain stably at the adjacent second mode. A tuning arrangement tunes the laser from mode to mode thereby to select each one of the plural optical channels. A conventional external optical amplitude modulator adds user traffic to a beam emitting from the laser to provide the information-carrying laser beam, and a coupler couples the traffic-carrying laser beam into an optical fiber of the optical telecommunications system. In the case of a SOA, the structure of the gain medium may lend itself to the integration of a modulator (e.g. electro absorptive or Mach-Zender)
The external cavity length is determined in accordance with the following parameters:
1) The frequency spacing of the ITU grid (or whole number fraction thereof) to be achieved.
2) The optical and/or temporal dispersion produced by intracavity optical elements.
3) The effective optical and/or temporal dispersion caused by the act of tuning.
In either a VECSEL or SOA based laser the external mirror is normally spherical and positioned relative to the semiconductor structure by a spacer structure mounted to the heat sink at a distance in a range of 0.5 mm and 50 mm to form the external cavity and in the present invention this distance is chosen to create a laser frequency comb corresponding to a predetermined optical channel spacing arrangement. Equivalently to the spherical mirror, a focusing intra cavity lens and piano plano mirror can be used to form the external cavity. The key to the external cavity design is that it be a stable resonator as defined in Siegman xe2x80x9cLasersxe2x80x9d University Science Books 1986 where the external cavity serves primarily as a feedback stabilization mechanism.
In accordance with some embodiments of the present invention, the VECSEL or ECSAL includes a heat sink structure and a semiconductor structure grown by molecular beam epitaxy upon a substrate and attached to the heat sink. As completed, the semiconductor structure comprises a multi-layer semiconductor or dialectic mirror region, a homogeneously broadened multiple quantum well gain region having a thickness equal to at least one design wavelength and having a plurality of quantum wells, each quantum well being optimally positioned with respect to a standing wave in the active gain region at the design wavelength, and an antireflection coating region having a low reflectance at the design wavelength. In the case of a MQW VECSEL the mirror can advantageously be a semiconductor Bragg reflector achieving at least 99% reflectance and the active region can be as short as one design wavelength. In the case of a SOA as the gain region, one mirror of the cavity can be a cleaved facet that may or may not have a dielectric coating and its active region can be hundreds of times the design wavelength, also in the case of a SOA, the active mirror is normally electrically pumped. However, careful consideration must be given to the waveguide design of the SOA to ensure that a good overlap between the external cavity mode and the fundamental waveguide mode is achieved and that higher order waveguide modes are not excited.
In the case of an optically pumped MQW VECSEL, an in-plane laser (e.g. Fabry Perot or DFB diode laser) providing pump radiation is aligned relative to an external surface of the semiconductor quantum well at Brewster""s angle relative to the axis of pump laser emission. In this embodiment the diode laser pump is a sub-assembly which is aligned and secured in a sidewall of the spacer structure and is thereby made integral therewith. A pump radiation absorption element or aperture is preferably formed in the spacer structure at Brewster""s angle opposite an angle of incidence of the pump radiation for absorbing any pump radiation residually reflected from the external surface of the active region. Note that by pumping the structure at Brewster""s angle, over 90% of the pump radiation is typically absorbed. An optimized pumping arrangement can deliver as much as 99% of the pump radiation to the VECSEL structure. Brewster""s angle is about 74 for InP based materials, which corresponds to a tan (74)=3:1 ratio for the incoming pump beam axes to produce a circular spot on the semiconductor. Because the geometry of most edge-emitting devices is 3:1, pumping at Brewster""s angle eliminates the need to circularize the pump laser beam, reducing the complexity of the pump optics. In fact, only focusing optics such as one or two lenses are required to image the pump beam to the correct spot size on the active region. A simple xcex/2 plate provides the correct polarization for incidence at Brewster""s angle and hence maximum absorption. As is known to those skilled skilled in the art, the term Brewster""s angle refers to the angle of incidence of light reflected from the surface of dielectric material at which the reflectivity for light whose electrical vector is in the plane of incidence becomes zero. This is sometimes referred to as the polarizing angle. In the context of the present invention where the dielectric material has a chemically composite multilayer structure and has in effect a composite refractive index, Brewster""s angle (sometimes referred to as an angle analogous to Brewster""s angle) is the angle corresponding to that composite refractive index. The size of the pump beam on the VECSEL is advantageously matched to the size of the external cavity transverse mode and accounts for thermal effects (such as thermal lensing) in the VECSEL itself.
A method for calibrating a stepwise tunable, external cavity amplifying mirror (STECAM) laser is also provided. The STECAM laser includes an external cavity formed by an external mirror at a length fixed by a spacer. The length defines a plurality of optical modes, each mode corresponding to a channel wavelength of an optical telecommunications system having plural optical channels. The semiconductor structure of the active mirror has a homogeneously broadened multiple quantum well (MQW) active region wherein the gain curve exceeds cavity losses over a band which is less than the mode-to-mode spacing, the laser output being tunable to step from a first mode to an adjacent second mode and to remain stably at the adjacent second mode. A tuning mechanism such as a thermoelectric cooler for cooling and heating the active gain region, or a frequency-selective element such as an intra-cavity etalon, is provided for tuning the STECAM laser from mode to mode. A digital controller including a wavelength-selective optical sensor responsive to laser output radiation tunes the laser from mode to mode and maintains the laser at each mode thereby to select each one of the plural optical channels. The wavelength-selective optical sensor generates pulses responsive to inter-mode optical transitions. The method includes steps of:
sweeping the tuning mechanism between a longest and a shortest wavelength mode capable of being generated by the laser in accordance with a control parameter generated by the digital controller,
recording in the memory of the digital controller a transition control parameter presently being put out by the wavelength-selective optical sensing means upon detection of pulses responsive to inter-mode optical transitions, and
determining and recording single mode set values as approximately half increment magnitudes between magnitudes of adjacent recorded transition control parameters.
Thus, the tuning mechanism precision required is reduced from having to land exactly on an ITU channel wavelength to simply creating a profile such that the laser snaps itself to the appropriate channel, whose wavelength has already been predetermined by the external cavity. Such a laser does not require expensive wavelength control circuitry.